Thermoplastic polyurethanes, polyureas and polyurethaneureas represent an important family of segmented block copolymer thermoplastic elastomers. They can be extruded, injection or compression molded or solution spun. They offer a broad range of physical properties and characteristics, including high tensile and tear strength, chemical and abrasion resistance, good processibility, and protective barrier properties. Depending on composition, i.e. on the volume fraction of the soft, elastomeric segments, these polymers can be soft, rubbery or hard and rigid materials. The hard segments of polyurethanes are composed of diisocyanate and a small molecule diol chain extender, while the soft segments are mostly low molecular weight polymeric diols. Similarly, polyureas or polyurethaneureas comprise diamines and a combination of diols and diamines, respectively, in addition to diisocyanate. Polymeric diols include polyester diols, polyether diols, and polydiene diols. The polyester component is prone to hydrolytic degradation, the polyether component does not have sufficient resistance to oxidative degradation, especially in vivo, and polydienes suffer from inadequate thermal and oxidative stability.
Polyurethanes are the most commonly used materials in the production of biomedical devices that come in contact with blood such as pacemakers, defibrillators, angioplasty balloons, surgical drains, dialysis devices, etc. However, polyurethanes generally exhibit insufficient long-term in vivo biostability due to oxidation of the polyether soft segment, especially when in contact with metals, which catalyze oxidative degradation. This deficiency, limits the use of polyurethanes for long-term applications.
Polyisobutylene (PIB)-based thermoplastic polyurethanes (TPUs) offer high thermal, oxidative, and hydrolytic stability, however, polyisobutylene polyurethanes exhibit insufficient mechanical properties.